In the recent years, there has been remarkable technological progress in reducing the size and weight of communication apparatuses such as cellular phones. As means of realizing the reduction, not only reduction in the number of constituent components and in size, but also components in which a plurality of functions are combined have been developed. Under these circumstances on the background, those in which a surface acoustic wave filter for use in an RF stage of a communication apparatus is provided with a balance-unbalance converting function, which is so-called a balun function have also been actively studied in the recent years. They have come into use in, mainly, GSM (Global System for Mobile communications), etc. Also some patent applications concerning surface acoustic wave filters provided with balance-unbalance converting functions of the above type have been filed.
FIG. 37 shows a surface acoustic wave filter disclosed in Patent Document 1 which has a balance-unbalance converting function having an impedance of 50Ω at an unbalanced signal terminal and an impedance of 200Ω at a balanced signal terminal. In the configuration in FIG. 37, in a longitudinally-coupled-resonator surface acoustic wave filter 401 having three interdigital transducers (hereinafter referred to as IDTs), one comb electrode of an IDT 403 positioned in the center is substantially symmetrically divided into two portions in a surface-acoustic-wave propagation direction. The divided portions are connected to balanced signal terminals 408 and 409, respectively, and left and right IDTs 402 and 404 whose polarities are inverted are connected to an unbalanced signal terminal 407.
This allows the above surface acoustic wave filter to have a balance-unbalance converting function, and the impedance at the balanced signal terminal can be set to be approximately four times the impedance at the unbalanced signal terminal.
In addition, Patent Document 2 discloses that an IDT of a float balance type has asymmetry with respect to the central portion, in a surface-acoustic-wave propagation direction in the IDT, which is an imaginary central axis perpendicular to the surface-acoustic-wave propagation.
Specifically, it describes 1) the distance between adjacent comb electrodes, 2) the ratio (hereinafter referred to as the duty) of an electrode finger width to the pitch of an IDT, 3) an IDT pitch, and the formation of narrow pitch electrode fingers to have asymmetric pitches.
Patent Document 1: Japanese Unexamined Patent Application Publication No. 11-097966 (Publication Date: Apr. 9, 1999)
Patent Document 2: Japanese Unexamined Patent Application Publication No. 2003-46369 (Publication Date: Feb. 14, 2003)